Various techniques for suppressing such interfering signals in radar receivers are known. These techniques include, for example, the coding of the transmitted radar pulses in time and/or frequency as well as the establishment of nulls in the radiation diagram of the receiving antenna which point in the directions of the jammers. The latter measure, of course, is effective only if the location of the source or sources of interfering signals is known and is well separated from the area to be scanned.
In a paper titled "Adaptive Circular Polarization" by Fred E. Nathanson, presented at the 1975 IEEE International Radar Conference in Washington, D.C., there has been described an adaptive canceler designed to minimize the effect of atmospheric clutter upon the incoming echo signals. According to that proposal the outgoing signals are transmitted with circular polarization and the incoming signals are processed in one of two parallel circuit branches, depending on whether they are derived from incident waves that are circularly polarized in the same sense as the outgoing signals or in the opposite sense. The auxiliary signals in the opposite-sense branch are applied to a compensator, including a homodyne detector and a homodyne modulator in cascade with each other, where they are shifted in phase and reduced in amplitude to match the estimated interfering component of the incoming main signal in the same-sense branch from which a cancellation signal emitted by the compensator is subtracted. The compensator also receives a feedback signal, derived from the purged incoming signal at the output of the subtractor, for a continuous updating of the cancellation signal with a certain hysteresis introduced by an integrator or low-pass filter which lies between the cascaded stages of the compensator.
As noted in the above-identified paper, atmospheric clutter consists essentially of more or less spherical particles such as raindrops tending to reflect the incident circularly polarized waves with a reversed sense of polarization. This, however, is not the case with sources of jamming signals whose mode of polarization, in fact, may change from time to time in an unpredictable manner.